Programmable bias circuit architecture for a digital data/clock receiver

ABSTRACT

Receiver architectures and related bias circuits for a data processor are provided. One embodiment of a receiver architecture for a computer processor includes a first linear receiver stage configured to receive a first input, a second input, and a first bias voltage. The first linear receiver stage is configured to generate a first differential output signal in response to a comparison between the first input and the second input. The first differential output signal has a specified programmable voltage swing that is influenced by the first bias voltage. The receiver architecture also includes a first programmable bias circuit coupled to the first linear receiver stage. The first programmable bias circuit is configured to generate the first bias voltage.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The subject matter disclosed herein is related to that disclosed in U.S.patent application Ser. No. 12/100,979, which issued as U.S. Pat. No.7,652,937, and U.S. patent application Ser. No. 12/100,996.

TECHNICAL FIELD

Embodiments of the subject matter described herein relate generally toelectronic and computing systems. More particularly, embodiments of thesubject matter relate to data and clock receiver circuits of the typefound in computer processors.

BACKGROUND

Computer systems and other electronic systems rely on the communicationof digital data. Synchronous Dynamic Random Access Memory (SDRAM)devices are commonly used in computer systems, and such SDRAM devicescooperate with processor devices to support data read and writeoperations. The JEDEC Solid State Technology Association publishesspecifications related to double data rate (DDR) SDRAM devices. Theexisting DDR specifications are the DDR2 and DDR3 specifications.According to DDR2/DDR3, the SDRAM memory cells transfer data on bothrising and falling clock edges. DDR2 devices support 4-bit or 8-bitoutput burst modes, while DDR3 devices support an 8-bit output burstmode.

In a read operation, a DDR2/DDR3 device generates a clock/strobe signal(referred to as DQS) and data signals (referred to as DQ). Each byteincludes eight single-ended DQ signals and one or two differential DQSsignals. In one operating mode, one differential DQS signal is used toclock the eight DQ signals in a byte. In another operating mode, onedifferential DQS signal is used to clock four DQ signals in a byte,while a second differential DQS signal is used to clock the other fourDQ signals in the byte. Ideally, the memory device outputs the DQ andDQS signals for a given byte in a time-aligned (e.g., edge-aligned)manner. However, the edges may no longer be aligned by the time the DQand DQS signals reach the processor. In other words, the DQ signals willtypically be skewed relative to one another when they arrive at theprocessor. Excessive skew in the DQ signals can make it difficult toclock all of them using one DQS signal because of an increasedlikelihood of sampling a DQ signal while it is transitioning betweenvalid states, and excessive skew limits the maximum DDR operating speed.

BRIEF SUMMARY

The above and other aspects may be carried out by an embodiment of abias circuit for a receiver architecture of a computer processor. Thebias circuit includes a comparator having a positive input node, anegative input node, and an output node for a current control voltage.The bias circuit also includes a variable current source coupled to theoutput node of the comparator. The variable current source is configuredto generate a current that is dependent on the current control voltage,and to generate an internal bias voltage at a bias voltage node. Thebias circuit also includes a receiver replica stage having an outputnode coupled to the negative input node of the comparator and having abias input node coupled to the bias voltage node of the variable currentsource. The receiver replica stage is configured to mimic operation of acounterpart stage in the receiver architecture by generating a replicaoutput at its output node. The bias circuit also includes a programmablereference voltage generator coupled to the positive input node of thecomparator. The programmable reference voltage generator is configuredto generate a reference voltage that causes the receiver replica stageto be properly biased. The comparator receives the reference voltage andthe replica output and generates the current control voltage based on acomparison of the reference voltage to the replica output.

The above and other aspects may be carried out by an embodiment of areceiver architecture for a computer processor. The receiverarchitecture includes a first linear receiver stage configured toreceive a first input, a second input, and a first bias voltage. Thefirst linear receiver stage is configured to generate a firstdifferential output signal in response to a comparison between the firstinput and the second input, the first differential output signal havinga specified voltage swing that is influenced by the first bias voltage.The receiver architecture also includes a first programmable biascircuit coupled to the first linear receiver stage, the firstprogrammable bias circuit being configured to generate the first biasvoltage.

The above and other aspects may be carried out by an embodiment of areceiver architecture for a computer processor. The receiverarchitecture includes a linear receiver stage configured to receive abinary differential input signal and a bias voltage. The linear receiverstage is configured to generate a binary differential output signal byshifting the binary differential input signal by a shift voltage that isinfluenced by the bias voltage. The receiver architecture also includesa programmable bias circuit coupled to the linear receiver stage, theprogrammable bias circuit being configured to generate the bias voltage.

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the detaileddescription. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the subject matter may be derived byreferring to the detailed description and claims when considered inconjunction with the following figures, wherein like reference numbersrefer to similar elements throughout the figures.

FIG. 1 is a schematic representation of one embodiment of a computersystem that incorporates the receiver technologies described herein;

FIG. 2 is a schematic representation of an arrangement of DQS and DQreceivers suitable for use with a computer system;

FIG. 3 is a diagram that depicts an ideal waveform without skewassociated with the operation of the arrangement shown in FIG. 2;

FIG. 4 is a high level block diagram of an embodiment of a DQ receiversuitable for use with a processor of a computer system;

FIG. 5 is a high level block diagram of an embodiment of a DQS receiversuitable for use with a processor of a computer system;

FIG. 6 is a high level block diagram of an embodiment of a programmingarchitecture suitable for use with the DQ receiver shown in FIG. 4 andthe DQS receiver shown in FIG. 5;

FIG. 7 is a circuit schematic of the first two stages, and

FIG. 8 is a circuit schematic of the final three stages, of anembodiment of a DQ receiver;

FIG. 9 is a diagram that depicts signals associated with the operationof the DQ receiver shown in FIG. 7 and FIG. 8;

FIG. 10 is a circuit schematic of the first two stages, and

FIG. 11 is a circuit schematic of the final stage, of an embodiment of aDQS receiver;

FIG. 12 is a diagram that depicts signals associated with the operationof the DQS receiver shown in FIG. 10 and FIG. 11;

FIG. 13 is a circuit schematic of an embodiment of a first bias circuitsuitable for use with a DQ/DQS receiver;

FIG. 14 is a circuit schematic of an embodiment of a second bias circuitsuitable for use with a DQ/DQS receiver; and

FIG. 15 is a circuit schematic of an embodiment of a third bias circuitsuitable for use with a DQS receiver.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the embodiments of the subject matter or theapplication and uses of such embodiments. As used herein, the word“exemplary” means “serving as an example, instance, or illustration.”Any implementation described herein as exemplary is not necessarily tobe construed as preferred or advantageous over other implementations.Furthermore, there is no intention to be bound by any expressed orimplied theory presented in the preceding technical field, background,brief summary or the following detailed description.

As used herein, a “node” means any internal or external reference point,connection point, junction, signal line, conductive element, or thelike, at which a given signal, logic level, voltage, data pattern,current, or quantity is present. Furthermore, two or more nodes may berealized by one physical element (and two or more signals can bemultiplexed, modulated, or otherwise distinguished even though receivedor output at a common mode).

The following description refers to elements or nodes or features being“connected” or “coupled” together. As used herein, unless expresslystated otherwise, “connected” means that one element/node/feature isdirectly joined to (or directly communicates with) anotherelement/node/feature, and not necessarily mechanically. Likewise, unlessexpressly stated otherwise, “coupled” means that oneelement/node/feature is directly or indirectly joined to (or directly orindirectly communicates with) another element/node/feature, and notnecessarily mechanically. Thus, although an illustrated circuitschematic might depict one exemplary arrangement of elements, additionalintervening elements, devices, features, or components may be present inan embodiment of the depicted subject matter.

When used herein in the context of a label or descriptor for a signal ora voltage level, the letter “X” indicates the negative or inverse of acounterpart signal or voltage level. For example, a signal labeled ABCX,ABCx, or ABC_(x) represents the inverted component of a signal labeledABC. Likewise, a differential signal can be identified using thecombination of two complementary signals, such as ABC and ABCX.

For the sake of brevity, conventional aspects of DRAM devices, computerprocessor architectures, transistor-based electronic circuits,data/clock receivers, and other functional aspects of the systems (andthe individual operating components of the systems) may not be describedin detail herein. Furthermore, the connecting lines shown in the variousfigures contained herein are intended to represent exemplary functionalrelationships and/or physical couplings between the various elements. Itshould be noted that many alternative or additional functionalrelationships or physical connections may be present in an embodiment ofthe subject matter.

FIG. 1 is a schematic representation of one embodiment of a computersystem 100 that incorporates the receiver technologies described herein.It should be noted that the embodiment of computer system 100 shown inFIG. 1 is exemplary, and that the receiver circuits discussed herein maybe implemented in a wide variety of electronic systems other than anyparticular computing platform or architecture discussed here. In theembodiment shown, computer system 100 includes a processor 102, which iscoupled to a plurality of memory modules 104. More particularly,processor 102 includes a memory control interface 106, which in turnincludes an input/output (I/O) circuit 108. Although FIG. 1 depicts I/Ocircuit 108 as being implemented within processor 102, other embodimentsmay utilize an I/O circuit (or portions thereof) that resides outside ofthe processor core, or possibly on a different circuit chip.

The embodiment of I/O circuit 108 implemented in memory controlinterface 106 may utilize the linear receiver and sampling receivercircuits described in more detail below for receiving clock signals anddata signals, respectively, from memory modules 104. In practice, I/Ocircuit 108 receives clock and data signals generated by memory modules104, and processes the data signals into voltage levels that representones and zeros in the domain of processor 102. Thus, memory controlinterface 106 is coupled to receive signals from memory modules 104 viaI/O circuit 108. Although not the subject of this description, I/Ocircuit 108 may also be suitably configured with transmitter circuits tosupport data writing operations.

In certain embodiments, memory modules 104 may be implemented andpackaged as dual inline memory modules (DIMMs). The memory modules 104and memory control interface 106 may conform to various specifications,such as the DDR2 SDRAM Specification and/or the DDR3 SDRAM Specification(published by JEDEC Solid State Technology Association). The exemplaryembodiments described herein are designed for compatibility with theexisting DDR2/DDR3 protocols and electrical requirements. It should beappreciated that the techniques, concepts, and technologies describedherein need not be limited to DDR2/DDR3 applications. These techniques,concepts, and technologies can be modified and varied as needed forcompatibility with other memory device protocols and electricalrequirements.

FIG. 2 is a schematic representation of an arrangement 200 of clock anddata receivers suitable for use with a computer system such as computersystem 100, and FIG. 3 is a diagram that depicts signals associated withthe operation of the arrangement 200. This particular example includes aDDR2/DDR3 (either type will suffice) memory element 202 that is coupledto a DQS receiver 204 and a plurality of DQ receivers 206. Thisarrangement 200 also includes a delay locked loop (DLL) 208 coupled toDQS receiver 204 and to DQ receivers 206. Arrangement 200 depicts apractical embodiment where memory element 202 outputs eight bits of data(in the form of eight single-ended DQ signals) concurrently with onedifferential clock/strobe signal (in the form of a non-inverted DQSsignal and a corresponding inverted DQS signal). Accordingly,arrangement 200 has eight DQ receivers 206—one for each bit of thebyte—and one DQS receiver 204. A processor in a computer system may haveany number of DQS and DQ receivers, depending upon its particularconfiguration. For example, one embodiment of a processor may utilize 64DQ receivers per channel (eight bytes per channel; eight bits per byte),and two channels, for a total of 128 DQ receivers. Of course, anembodiment may be suitably configured to support any number of bits perbyte, any number of bytes, and any number of channels per processor.

In operation, DQS receiver 204 receives the differential DQS signal(which conveys a binary clock signal having a low voltage level and ahigh voltage level) from memory element 202 and generates a binarydifferential output 210 having voltage levels that are compatible withDLL 208. DLL 208 receives the differential output 210 from DQS receiver204, and generates therefrom a clock (CK) signal and a negative clock(NCK) signal by doubling the frequency of differential output 210 andadding delay. As mentioned previously, memory element 202 concurrentlygenerates the differential DQS signal and eight DQ signals. As depictedin FIG. 2, each DQ signal serves as an input to its respective DQreceiver 206, and the CK and NCK signals are used by DQ receivers 206 toprocess their respective DQ signals. Ideally, the output of a DQreceiver 206 will be a voltage corresponding to a logic high (i.e., aone) if the input DQ signal is greater than a reference voltage(MemVref) by any amount, and a voltage corresponding to a logic low(i.e., a zero) otherwise.

FIG. 3 illustrates an ideal scenario where the bits conveyed by the DQsignal are edge-aligned with the DQS signal. In practice, the memorydevice originates the differential DQS signal and the DQ signals in asubstantially edge-aligned manner, although such edge-alignment may besomewhat skewed when the signals are actually received at the processor.For simplicity, the inverse of the DQS signal is not shown in FIG. 3.Notably, for DDR2/DDR3, the DQ signal transitions at both the risingedge and the falling edge of the DQS signal. The DLL 208 generates theCK and NCK signals such that the rising edge of the CK signal (and,conversely, the falling edge of the NCK signal) is aligned near thecenter of the eye defined by the DQ signal. FIG. 3 illustrates thistiming alignment. In other words, the DLL 208 aligns the CK and NCKsignals to facilitate sampling of the DQ signal (and/or sampling ofdifferential signals generated during the processing of the DQ signal)at a well-defined and settled time, rather than near one of the twotransition points. As mentioned above, the timing of the CK and NCKsignals is important for practical embodiments where multiple DQ signals(for example, eight) are clocked by a common DQS signal. Thus, atransition edge of the CK/NCK signal preferably occurs at a samplingtime when all of the DQ signals are well-settled. This enablesmaximizing of DDR speed. At low speeds, the placement of the DQS signalis relatively flexible since the sampling eye is wider. However, athigher speeds the sampling eye is narrower and finding the optimalCK/NCK edges can be a major factor in determining the DDR speed.

FIG. 4 is a high level block diagram of an embodiment of a DQ receiver300 suitable for use with a processor of a computer system. DQ receiver300 is preferably implemented as an integrated circuit that ismanufactured using an appropriate semiconductor fabrication process,such as a 65 nanometer silicon-on-insulator (SOI) process. Each of theDQ receivers implemented in an I/O circuit of a computer processor canbe configured in this manner.

DQ receiver 300 receives as inputs a respective single-ended data inputsignal (labeled DQ_IN) and a reference voltage (labeled MemVref), andgenerates an output signal in response to DQ_IN. In practice, the DQ_INsignal represents a signal obtained from a memory element, e.g., a DDR2or DDR3 memory device, where that signal conveys binary information(i.e., logic high and logic low voltage levels). In preferredembodiments the MemVref voltage is common among all of the DQ receiversemployed by the processor. Notably, DQ_IN and MemVref can be of anyvoltage ranging from VSS to VDDIO (as defined in the applicable DDRspecification), including voltages that exceed the stated oxidebreakdown threshold of the transistors utilized to implement DQ receiver300. As described in more detail below, DQ receiver 300 is suitablyconfigured and programmed to provide overvoltage protection for thetransistors by scaling its operating voltage characteristics such thatthe oxide breakdown threshold is not actually exceeded.

The output signal of DQ receiver 300 is generated in a voltage domainthat is appropriate for the processor, and with the desired voltageswing between logic high and logic low levels. In practice, DQ receiver300 needs to be able to detect and resolve voltage swings that may notbe as large as that generated by the memory elements. The receivedsignals may also be corrupted in time, have distorted wave shapes, orthe like, and DQ receiver 300 is preferably configured to handle suchinconsistencies.

DQ receiver 300 is preferably configured as a multistagetransistor-based circuit. This particular embodiment includes fivestages, although more or less may be possible in different embodiments.Each of the stages is described in more detail below with reference toexemplary transistor-based circuit implementations. DQ receiver 300includes a linear receiver portion and a clocked sense amplifier portionthat is directly connected to the linear receiver portion. In FIG. 4,the linear receiver portion includes a first stage 302 combined with asecond stage 304, and the clocked sense amplifier portion includes athird stage 306 combined with a fourth stage 308.

Notably, DQ receiver 300 includes programmable operating characteristicsand features that allow it to flexibly accommodate manufacturing processvariations. Such programmability is desirable such that the circuitry ofDQ receiver 300 need not be redesigned in response to variations,developments, or changes in the particular semiconductor fabricationprocess. In FIG. 4, DQ receiver 300 is generally depicted with aprogramming architecture 309, which is suitably configured to program,control, or otherwise influence the operation of DQ receiver 300. In apractical implementation, programming architecture 309 may include orcooperate with, without limitation: one or more bias circuits; one ormore memory elements; a computer-executable software program, e.g., aBIOS of the host computing platform; or the like. Moreover, althoughprogramming architecture 309 is depicted as a single functionalcomponent in FIG. 4, it may actually be realized using any number ofelements, circuits, functional modules, etc. For the particularembodiment shown in FIG. 4, first stage 302 and second stage 304 areprogrammable elements that can be controlled in response to settingsestablished by programming architecture 309. The programmable nature offirst stage 302 and second stage 304 will be described in more detailbelow.

The first stage 302 has an input node for receiving DQ_IN, and an inputnode for receiving the reference voltage (MemVref). First stage 302includes output nodes for a differential output signal that is generatedin response to a comparison of DQ_IN to MemVref. In this regard, ifDQ_IN is greater than MemVref, then the non-inverted output (labeledDQ_OUT1) will be set at a designated high output voltage and theinverted output (labeled DQ_OUT1X) will be set at the designated highoutput voltage minus a designated swing voltage, which is programmablevia programming architecture 309. On the other hand, if DQ_IN is notgreater than MemVref, then DQ_OUT1X will be at the designated highoutput voltage and DQ_OUT1 will be at the designated high output voltageminus the designated swing voltage. Conceptually, DQ_OUT1 “tracks” DQ_INin that it is relatively high when DQ_IN is high, and it is relativelylow when DQ_IN is low.

The second stage 304 of DQ receiver 300 is coupled to first stage 302.Second stage 304 receives DQ_OUT1 and DQ_OUT1X as inputs. Second stage304 has a pair of level shifters that reduces the voltage of DQ_OUT1 andDQ_OUT1X by a predetermined shift voltage, which is programmable viaprogramming architecture 309. Thus, second stage 304 generates adifferential output (labeled DQ_OUT5 and DQ_OUT5X), where:DQ_OUT5=DQ_OUT1−V _(SHIFT) _(—) _(DQ); andDQ_OUT5X=DQ_OUT1X−V _(SHIFT) _(—) _(DQ).The value of V_(SHIFT) _(—) _(DQ) is selected to make DQ_OUT5 andDQ_OUT5X compatible with the subsequent stages of DQ receiver 300.

A third stage 306 and a fourth stage 308 of DQ receiver 300 function asa sense amplifier that converts a relatively small swing dual railsignal into a usable digital signal that has voltage levels compatiblewith the processor. Third stage 306 has input nodes that are coupled tothe output nodes of second stage 304 of DQ receiver 300. Third stage 306uses DQ_OUT5 and DQ_OUT5X as inputs, and generates two binary outputsignals (labeled OUT8 and OUT8X) in response thereto. As depicted inFIG. 4, third stage 306 is sampled by CK. Fourth stage 308, which iscoupled to third stage 306, uses OUT8 and OUT8X as inputs, and generatestwo binary output signals (labeled OUT9 and OUT9X) in response thereto.As explained in more detail below, OUT9 represents a binary outputsignal having voltage characteristics (e.g., voltage swing and voltagelevel) that are compatible with the computer processor. Fourth stage 308is sampled by NCK, which is the inverse of the CK signal. The embodimentshown in FIG. 2 generates the CK and NCK signals with DLL 208.

Third stage 306 and fourth stage 308 cooperate to transform thedifferential signal (DQ_OUT5 and DQ_OUT5X) into the binary output signallabeled OUT9. In turn, a fifth stage 310 of DQ receiver 300 is coupledto fourth stage 308. As shown in FIG. 5, fifth stage 310 operates inresponse to the NCK signal. Fifth stage 310 is suitably configured tofunction as a “zero catcher” or a “pulse stretcher” that receives OUT9as an input, and transforms OUT9 into a respective digital output signal(labeled OUT) that conveys valid data throughout entire clock cycles ofDQ receiver 300 (i.e., cycles of CK and/or NCK). Fifth stage 310 may benecessary in practical implementations where OUT9 is valid only for halfof a clock cycle. The OUT signal conveys bits using the voltage levelsand voltage swing utilized by the processor. Note that the OUT9X signalneed not be utilized by DQ receiver 300. In practice, OUT9X may serve asan input to a load matching circuit, element, or stage (not shown) forpurposes of balancing the output of fourth stage 308.

FIG. 5 is a high level block diagram of an embodiment of a DQS receiver400 suitable for use with a processor of a computer system. DQS receiver400 is preferably implemented as an integrated circuit that ismanufactured using an appropriate semiconductor fabrication process,such as a 65 nanometer SOI oxide process. Moreover, DQS receiver 400 canbe integrally fabricated with DQ receivers 300, and DQS receiver 400 andDQ receivers 300 can be used together in one DDR interface. Each of theDQS receivers implemented in an I/O circuit of a computer processor canbe configured in the manner depicted in FIG. 5.

DQS receiver 400 receives a differential clock/strobe signal (labeledDQS_IN and DQS_INX) as an input, and generates a differential outputsignal (labeled DQS_OUT7 and DQS_OUT7X) in response thereto. Inpractice, the DQS_IN and DQS_INX signals are obtained from a memoryelement, e.g., a DDR2 or DDR3 memory device, and the DQS_IN and DQS_INXsignals are associated with a plurality of DQ signals obtained from thesame memory element. Moreover, DQS_OUT7 and DQS_OUT7X are generated in avoltage domain that is appropriate for the DLL (see FIG. 2) thatproduces the CK and NCK signals utilized by DQ receiver 300. Notably,DQS_IN and DQS_INX can be of any voltage ranging from VSS to VDDIO (asdefined in the applicable DDR specification), including voltages thatexceed the stated oxide breakdown threshold of the transistors utilizedto implement DQS receiver 400. As described in more detail below, DQSreceiver 400 is suitably configured and programmed to provideovervoltage protection for the transistors by scaling its operatingvoltage characteristics such that the oxide breakdown threshold is notactually exceeded.

DQS receiver 400 is preferably configured as a multistagetransistor-based circuit. This particular embodiment includes threestages (a first linear receiver stage 402, a second linear receiverstage 404, and a third linear receiver stage 406), although more or lessmay be possible in different embodiments. Each of the stages isdescribed in more detail below with reference to exemplarytransistor-based circuit implementations.

Notably, DQS receiver 400 includes programmable operatingcharacteristics and features that allow it to flexibly accommodatemanufacturing process variations. Such programmability is desirable suchthat the circuitry of DQS receiver 400 need not be redesigned inresponse to variations, developments, or changes in the particularsemiconductor fabrication process. In FIG. 5, DQ receiver 400 isgenerally depicted with a programming architecture 408, which issuitably configured to program, control, or otherwise influence theoperation of DQS receiver 400. In a practical implementation,programming architecture 408 may include or cooperate with, withoutlimitation: one or more bias circuits; one or more memory elements; acomputer-executable software program, e.g., a BIOS of the host computingplatform; or the like. In this regard, programming architecture 408 maybe combined with programming architecture 309 (FIG. 4) in variousembodiments. Moreover, although programming architecture 408 is depictedas a single functional component in FIG. 5, it may actually be realizedusing any number of elements, circuits, functional modules, etc. For theparticular embodiment shown in FIG. 5, first linear receiver stage 402,second linear receiver stage 404, and third linear receiver stage 406are programmable elements that can be controlled in response to settingsestablished by programming architecture 408. The programmable nature ofthese stages will be described in more detail below.

The first linear receiver stage 402 is configured to receive DQS_IN andDQS_INX as inputs, and first linear receiver stage 402 transforms DQS_INand DQS_INX into a differential output signal (labeled DQS-OUT1 andDQS_OUT1X) that corresponds to a version of the differential DQS inputhaving a desired voltage swing, which is programmable via programmingarchitecture 408. The second linear receiver stage 404 of DQS receiver400 is coupled to first linear receiver stage 402. Second linearreceiver stage 404 receives DQS_OUT1 and DQS_OUT1X as inputs, and secondlinear receiver stage 404 functions as a level shifter that reduces thevoltage of DQS_OUT1 and DQS_OUT1X by a predetermined shift voltage,which is programmable via programming architecture 408. Thus, secondlinear receiver stage 404 generates a differential output (labeledDQS_OUT5 and DQS_OUT5X), where:DQS_OUT5=DQS_OUT1−V _(SHIFT) _(—) _(DQS); andDQS_OUT5X=DQS_OUT1X−V _(SHIFT) _(—) _(DQS).In certain embodiments, the circuit topology of second linear receiverstage 404 is identical to the circuit topology of second stage 304 of DQreceiver 300 (as described in more detail below). Moreover, for thisparticular implementation, V_(SHIFT) _(—) _(DQS) equals V_(SHIFT) _(—)_(DQ), which is associated with the operation of DQ receiver 300.

The third linear receiver stage 406 of DQS receiver 400 is coupled tosecond linear receiver stage 404. Third linear receiver stage 406receives DQS_OUT5 and DQS_OUT5X as inputs. Third linear receiver stage406 is a differential amplifier that transforms DQS_OUT5 and DQS_OUT5Xinto a differential output signal (labeled DQS_OUT7 and DQS_OUT7X)having appropriate voltage levels and a designated voltage swing thatare compatible with the DLL of the processor (see FIG. 2). The voltageswing of third linear receiver stage 406 is also programmable viaprogramming architecture 408. As mentioned above, the clock signals usedby the clocked sense amplifier portion of the DQ receivers are derivedfrom DQS_OUT7 and DQS_OUT7X, with two times frequency multiplication.

FIG. 6 is a high level block diagram of an embodiment of a programmingarchitecture 500 suitable for use with DQ receiver 300 and DQS receiver400. FIG. 6 depicts a simplified high level arrangement of a biascircuit 502, a bias circuit 504, a bias circuit 506, a program memoryelement 508 for bias circuit 502, a program memory element 510 for biascircuit 504, and a program memory element 512 for bias circuit 506. Eachof these bias circuits is described in more detail below with referenceto exemplary transistor-based circuit implementations. Bias circuit 502is suitably configured to generate a bias voltage (labeled BIAS1)voltage used by first stage 302 of DQ receiver 300 and first linearreceiver stage 402 of DQS receiver 400. Bias circuit 504 is suitablyconfigured to a bias voltage (labeled BIAS5) used by second stage 304 ofDQ receiver 300 and second linear receiver stage 404 of DQS receiver400. In certain embodiments, bias circuit 504 is coupled to bias circuit502 such that bias circuit 504 can receive one or more inputs from biascircuit 502. Bias circuit 506 is suitably configured to generate a biasvoltage (labeled BIAS7) used by third linear receiver stage 406 of DQSreceiver 400.

As described in more detail below, the bias circuits are programmable toaccommodate different PVT (manufacturing Process, supply Voltage, andoperating Temperature) combinations and conditions. In addition, thebias circuits are operable to set the various swing voltages and shiftvoltages described here with reference to the DQ and DQS receivers. Incertain embodiments, program memory elements 508/510/512 are suitablyconfigured to store instructions, binary codes, register values,settings, or other information that is utilized to program the biascircuits. Although each program memory element is depicted as a separatefunctional component in FIG. 6, any two or all of the program memoryelements can be combined. In preferred embodiments, each of the programmemory elements 508/510/512 includes or cooperates with a one or moreregisters associated with the basic input/output system (BIOS) of theprocessor, and the BIOS can be rewritten as needed to update the programsettings of bias circuit 502, bias circuit 504, and/or bias circuit 506.

The DQ receivers, DQS receivers, and bias circuits described above canbe implemented using transistor-based electronic circuits (e.g., NMOSand PMOS transistors) manufactured by a suitable semiconductorfabrication process. In this regard, FIG. 7 is a circuit schematic ofthe first two stages, and FIG. 8 is a circuit schematic of the finalthree stages, of an embodiment of a DQ receiver 600. DQ receiver 600includes a first stage 602, a second stage 604, a third stage 606, afourth stage 608, and a fifth stage 610. The general configuration andfunctionality of DQ receiver 600 is consistent with that described abovefor DQ receiver 300 (FIG. 4), and common features and aspects will notbe redundantly described in detail here.

First stage 602 utilizes NMOS transistors configured as a common sourcecascode linear amplifier with resistor loading. In particular, firststage 602 includes, without limitation: four transistors (referencenumbers 612, 614, 616, and 618), a load resistor 620 coupled between thedrain of transistor 612 and a supply voltage (VDDIO) node, and a loadresistor 622 coupled between the drain of transistor 614 and the supplyvoltage node. First stage 602 also includes a tail current source 624,which may be realized as one or more transistors.

The VDDIO voltage at the supply voltage node represents the primary(upper) supply voltage used by the I/O circuit of the processor. ForDDR2, the VDDIO voltage is nominally 1.8 volts; for DDR3, the VDDIOvoltage is nominally 1.5 volts. In other embodiments, VDDIO may be moreor less than that required by the DDR2/DDR3 Specification. The exampledescribed below uses 2.0 volts for VDDIO as a convenient value. Loadresistors 620/622 are selected with consideration of the currentgenerated by the transistors in first stage 602, the value of VDDIO, theload capacitance, and the desired voltage range of DQ-OUT1 and DQ_OUT1X.As one non-limiting example, load resistors 620/622 are each 2.4 kΩ inthe illustrated embodiment.

The drain of transistor 612 is coupled to load resistor 620, the gate oftransistor 612 is coupled to receive a DC bias voltage (labeled VCAS),and the source of transistor 612 is coupled to the drain of transistor616. Similarly, the drain of transistor 614 is coupled to load resistor622, the gate of transistor 614 is coupled to receive VCAS, and thesource of transistor 614 is coupled to the drain of transistor 618. VCASis generated by the associated bias circuitry, and is also used in theDQS receivers. In operation, the bias circuitry determines anappropriate voltage for VCAS, depending upon the particular PVT corner.In other words, VCAS is programmable via the appropriate programmingarchitecture. First stage 602 generates a differential output at thenodes corresponding to the drains of transistor 612 and transistor 614.More specifically, the branch that includes transistor 614 generatesDQ_OUT1, and the branch that includes transistor 612 generates DQ_OUT1X.

The drain of transistor 616 is coupled to the source of transistor 612,the gate of transistor 616 is coupled to receive the single-ended DQ_INsignal, and the source of transistor 616 is coupled to the source oftransistor 618. The drain of transistor 618 is coupled to the source oftransistor 614, the gate of transistor 618 is coupled to receive theMemVref reference voltage, and the source of transistor 618 is coupledto the source of transistor 616. The gate of transistor 616 receives theDQ_IN signal from the associated memory device, and the gate oftransistor 618 receives the MemVref voltage, which is generated by anexternal supply with an external ground reference. This MemVref voltageis used to slice DQ_IN. The value of MemVref is typically chosen to behalf of VDDIO.

Tail current source 624 is coupled between the common source oftransistors 616/618 and ground. Tail current source 624 receives theBIAS1 voltage and responds thereto to bias first stage 602 in anappropriate manner.

First stage 602 functions such that it produces dual-rail outputs of afixed voltage swing from the positive supply voltage rail (VDDIO),regardless of the waveform characteristics (magnitude, rising/fallingedge rate, crossover voltage, etc.) of DQ_IN. The output voltage swingis controlled by the BIAS1 voltage via tail current source 624 and loadresistors 620/622. In operation, when DQ_IN exceeds MemVref by anyamount, transistor 616 turns on and transistor 618 turns off, resultingin a voltage drop across load resistor 620 and no voltage drop acrossload resistor 622. Thus, first stage 602 will generate DQ_OUT1 andDQ_OUT1X to indicate a logic high value. For this embodiment, DQ_OUT1transitions to a relatively high voltage when DQ_IN transitions to avoltage that exceeds MemVref, and transitions to a relatively lowvoltage when DQ_IN transitions to a voltage that does not exceedMemVref.

FIG. 9 is a diagram that depicts signals associated with the operationof DQ receiver 600. The upper portion (identified by reference number626) of FIG. 9 is based on common vertical voltage scale. Forconvenience, VDDIO equals 2.0 volts, MemVref equals 1.0 volt, and VSSequals 0.0 volts in this example. The point 628 represents the time whenDQ_IN crosses the MemVref threshold. In response to this crossing,DQ_OUT1 and DQ_OUT1X transition after a slight delay caused by thepractical operating characteristics of first stage 602. The point 630represents the time when DQ_IN falls below the MemVref threshold. Inresponse to this crossing, DQ_OUT1 and DQ_OUT1X again transition after aslight delay. Notably, first stage 602 will generate an output thatrepresents a logic high value whenever DQ_IN is greater than MemVref(regardless of how much greater), and first stage 602 will generate anoutput that represents a logic low value whenever DQ_IN is less thanMemVref (regardless of how much less).

Referring to FIG. 9, DQ_OUT1 and DQ_OUT1X swing between a relativelyhigh voltage and a relatively low voltage that do not change duringnormal operation of the DQ receiver. For this particular embodiment, therelatively high voltage level for DQ_OUT1 and DQ_OUT1X is equal to VDDIO(2.0 volts in this example), while the relatively low voltage level forDQ_OUT1 and DQ_OUT1X is equal to VDDIO minus a desired swing voltage(V_(SWING) _(—) _(DQ1)). This example assumes that V_(SWING) _(—) _(DQ1)equals 0.4 volts. Accordingly, the relatively low voltage level forDQ_OUT1 and DQ_OUT1X will be 1.6 volts for this example. As described inmore detail below, the bias circuitry for first stage 602 is suitablyconfigured to control the value of V_(SWING) _(—) _(DQ1), whichrepresents the nominal expected voltage drop across load resistors620/622 during operation. This scheme ensures that V_(SWING DQ1) ismaintained regardless of the actual value of VDDIO and regardless ofvariation between load resistors 620/622 or manufacturing processvariations.

Second stage 604 of DQ receiver 600 is suitably configured to functionas a pair of level shifters that downward shift the dual-rail outputs offirst stage 602 (DQ_OUT1 and DQ_OUT1X) for compatibility with thirdstage 606. For embodiments that support both DDR2 and DDR3, the voltageshift introduced by second stage 604 will be different for DDR2 andDDR3. Referring to FIG. 7, this embodiment of second stage 604 includes,without limitation: two PMOS transistors 632/634; eight NMOS transistors(reference numbers 636, 638, 640, 642, 643, 645, 647, and 649); a firsttail current source 644 that includes transistors 643 and 647; and asecond tail current source 646 that includes transistors 645 and 649.One section of second stage 604 is configured to shift the non-invertedcomponent (DQ_OUT1) by the designated shift voltage, and another sectionof second stage 604 is configured to shift the inverted component(DQ_OUTX) by the same designated shift voltage.

The source of transistor 632 is coupled to the VDDIO supply, the gate oftransistor 632 is coupled to receive a control voltage (labeledDDR3_TDX), and the drain of transistor 632 is coupled to the drain oftransistor 638. The drain of transistor 636 is coupled to the VDDIOsupply, the gate of transistor 636 receives DQ_OUT1 from first stage602, and the source of transistor 636 is coupled to a node 637. Thedrain of transistor 638 is coupled to the drain of transistor 632, thegate of transistor 638 also receives DQ_OUT1 from first stage 602, andthe source of transistor 638 is coupled to the node 637.

The source of transistor 634 is coupled to the VDDIO supply, the gate oftransistor 634 is coupled to receive DDR3_TDX, and the drain oftransistor 634 is coupled to the drain of transistor 642. The drain oftransistor 640 is coupled to the VDDIO supply, the gate of transistor640 receives DQ_OUT1X from first stage 602, and the source of transistor640 is coupled to a node 641. The drain of transistor 642 is coupled tothe drain of transistor 634, the gate of transistor 642 also receivesDQ_OUT1X from first stage 602, and the source of transistor 642 iscoupled to the node 641.

The drain of transistor 643 is coupled to the node 637, the gate oftransistor 643 receives a DC voltage (VTT, which is nominally half ofVDDIO in this example), and the source of transistor 643 is coupled tothe drain of transistor 647. Similarly, the drain of transistor 645 iscoupled to the node 641, the gate of transistor 645 receives VTT, andthe source of transistor 645 is coupled to the drain of transistor 649.As shown, transistor 647 is coupled between transistor 643 and ground,while transistor 649 is coupled between transistor 645 and ground. Inthis embodiment, tail current source 644 is configured as a cascodearrangement of transistors 643/647, and tail current source 646 isconfigured as a cascode arrangement of transistors 645/649. Notably,transistors 643/645 provide over-voltage protection for transistors647/649, because they prevent the drain of transistor 647 and the drainof transistor 649 from reaching VDDIO (which could cause thegate-to-drain voltage to exceed the gate oxide breakdown voltage). Tailcurrent source 644 receives the BIAS5 voltage and responds thereto tobias the respective portion of second stage 604 in an appropriatemanner. Similarly, tail current source 646 also receives the BIAS5voltage and responds thereto to bias the respective portion of secondstage 604 in an appropriate manner.

Second stage 604 functions to downwardly shift DQ_OUT1 and DQ_OUT1X intolower voltage equivalents DQ_OUT5 and DQ_OUT5X, respectively. Theassociated bias circuitry influences, dictates, and controls the shiftvoltage introduced by second stage 604. As mentioned above, the voltageshift amount for DDR2 devices is different than the voltage shift amountfor DDR3 devices (the desired voltage shift for DDR3 is less than thedesired voltage shift for DDR2). Accordingly, this embodiment utilizestransistors 632/634 to switch between operation in a DDR2 mode or a DDR3mode. For this example, when DDR3_TDX is high, transistor 632 andtransistor 634 turn on, which in turn causes the voltage shift reflectedin DQ_OUT5 and DQ_OUT5X to be less, compared to when DDR3_TDX is low.

The output signal DQ_OUT5 is taken at node 637, i.e., the common sourcenode of transistors 636/638, and the output signal DQ_OUT5X is taken atnode 641, i.e., the common source node of transistors 640/642. Referringagain to FIG. 9, DQ_OUT5 equals DQ_OUT1 minus the designated shiftvoltage (V_(SHIFT) _(—) _(DQ2)), and DQ_OUT5X equals DQ_OUT1X minusV_(SHIFT) _(—) _(DQ2). This example assumes that V_(SHIFT) _(—) _(DQ2)equals 0.6 volts. Accordingly, the relatively high voltage level forDQ_OUT5 and DQ_OUT5X will be VDDIO minus V_(SHIFT) _(—) _(DQ2), or 1.4volts for this example. Similarly, the relatively low voltage level forDQ_OUT5 and DQ_OUT5X will be VDDIO minus V_(SWING) _(—) _(DQ1) minusV_(SHIFT) _(—) _(DQ2), or 1.0 volts for this example. Notably, secondstage 604 preserves V_(SWING) _(—) _(DQ1) between DQ_OUT5 and DQ_OUT5X(0.4 volts in this example). As described in more detail below, the biascircuit architecture for DQ receiver 600 is suitably configured to biasthe linear receiver portion with BIAS1 and BIAS5 such that thedifferential output signal (DQ_OUT5 and DQ_OUT5X) swings between thespecified upper voltage level and the specified lower voltage level.

Third stage 606 and fourth stage 608 function as back-to-back samplingamplifier stages, where third stage 606 is configured as a first senseamplifier and fourth stage 608 is configured as a second senseamplifier. Third stage 606 is clocked by the non-inverted CK signal, andfourth stage 608 is clocked by the inverted NCK signal to reducemetastability issues. The use of clocked sense amplifiers in this mannerprovides benefits over existing approaches that use non-clocked senseamplifiers. Notably, third stage 606 and fourth stage 608 reside in DQreceiver 600 itself to reduce DQ-to-DQ skew between signals clocked by acommon DQS signal. Such skew would otherwise be caused by buffering thereceiver output at the analog-to-digital interface and by routemismatches associated with the DQ receivers processing a byte, or byroute mismatches associated with the DQS receiver used for the byte.Incorporating third stage 606 and fourth stage 608 into DQ receiver 600and the analog front end stages enables DQ_OUT5 and DQ_OUT5X to beimmediately sensed and converted into a usable digital signal.

This embodiment of third stage 606 includes, without limitation: twoPMOS transistors 648/650; four NMOS transistors (reference numbers 652,654, 656, and 658), and an NMOS transistor 660 that receives the CKsignal. The source of transistor 648 is coupled to VDDIO, the gate oftransistor 648 is coupled to a node 662, and the drain of transistor 648is coupled to a node 664. One output of third stage 606 represents afirst sampled output signal (labeled OUT8X—provided at node 662), andanother output of third stage 606 represents a second sampled outputsignal (labeled OUT8—provided at node 664). The source of transistor 650is coupled to VDDIO, the gate of transistor 650 is coupled to node 664,and the drain of transistor 650 is coupled to node 662.

The drain of transistor 652 is coupled to node 664, the gate oftransistor 652 is coupled to node 662, and the source of transistor 652is coupled to the drain of transistor 656. Similarly, the drain oftransistor 654 is coupled to node 662, the gate of transistor 654 iscoupled to node 664, and the source of transistor 654 is coupled to thedrain of transistor 658. The drain of transistor 656 is coupled to thesource of transistor 652, the gate of transistor 656 receives DQ_OUT5Xfrom second stage 604, and the source of transistor 656 is coupled tothe source of transistor 658. Likewise, the drain of transistor 658 iscoupled to the source of transistor 654, the gate of transistor 658receives DQ_OUT5 from second stage 604, and the source of transistor 658is coupled to the source of transistor 656. The drain of transistor 660is coupled to the common source node of transistors 656/658, the gate oftransistor 660 receives the CK signal, and the source of transistor 660is grounded.

Third stage 606 is suitably configured to generate DQ_OUT8 and DQ_OUT8Xin response to DQ_OUT5, DQ_OUT5X, and the CK signal. In operation, thirdstage 606 is precharged when CK is low and NCK is high. In other words,node 662 and node 664 (and, in turn, OUT8 and OUT8X) are precharged to adesignated voltage level (referred to herein as VTT). Thus, third stage606 samples its input on the rising edge of CK, and it maintains itsoutput on nodes 662/664 until the falling edge of CK, when OUT8 andOUT8X get precharged to VTT. For this embodiment, VTT is the mid-railsupply voltage level, which is typically half the VDDIO voltage. When CKtransitions from low-to-high, third stage 606 begins to quickly evaluateits inputs (DQ_OUT5 and DQ_OUT5X) and, in response, adjust its outputs(OUT8 and OUT8X). On the other hand, when CK transitions fromhigh-to-low, OUT8 and OUT8X both transition to the relatively high VTTvoltage. For example, when CK transitions from low-to-high, if DQ_OUT5is greater than DQ_OUT5X, then third stage 606 will cause the voltage atnode 662 (OUT8X) to be pulled down to ground, while the voltage at node664 (OUT8) will remain at VTT. The converse applies if DQ_OUT5X isgreater than DQ_OUT5 when CK transitions from low-to-high. The exemplaryCK, NCK, OUT8, and OUT8X signals in FIG. 9, which share a common timeaxis with the upper portion 626 of FIG. 9, illustrate the operation ofthird stage 606. For this example, VTT is equal to 1.0 volt.

Referring again to FIG. 7, the desired values of V_(SWING) _(—) _(DQ1)and V_(SHIFT) _(—) _(DQ2) are dictated by the operating characteristicsand preferences of third stage 606. In this regard, for different PVTcombinations the bias circuit architecture calculates the optimal commonmode voltage for third stage 606 and the optimal input voltage swing forthird stage 606, where the common mode voltage is the average of the twoinputs (DQ_OUT5 and DQ_OUT5X) of third stage 606. In practice, the biascircuit architecture generates appropriate bias voltages for DQ receiver600 that facilitate optimal operation of third stage 606 over differentanticipated PVT corners. Again, the example described here assumes thatVDDIO equals 2.0 volts, VTT equals 1.0 volt, V_(SWING) _(—) _(DQ1)equals 0.4 volts, and V_(SHIFT) _(—) _(DQ2) equals 0.6 volts.

The output of third stage 606 serves as an input to fourth stage 608.This embodiment of fourth stage 608 includes, without limitation: twoPMOS transistors 666/668; four NMOS transistors (reference numbers 670,672, 674, and 676), and an NMOS transistor 678 that receives the NCKsignal. The source of transistor 666 is coupled to VDDIO, the gate oftransistor 666 is coupled to a node 680, and the drain of transistor 666is coupled to a node 682. One output of fourth stage 608 represents abinary output signal (labeled OUT9—provided at node 680), and anotheroutput of fourth stage 608 (labeled OUT9X) is provided at node 682. Thesource of transistor 668 is coupled to VDDIO, the gate of transistor 668is coupled to node 682, and the drain of transistor 668 is coupled tonode 680.

The drain of transistor 670 is coupled to node 682, the gate oftransistor 670 is coupled to node 680, and the source of transistor 670is coupled to the drain of transistor 674. Similarly, the drain oftransistor 672 is coupled to node 680, the gate of transistor 672 iscoupled to node 682, and the source of transistor 672 is coupled to thedrain of transistor 676. The drain of transistor 674 is coupled to thesource of transistor 670, the gate of transistor 674 receives OUT8 fromthird stage 606, and the source of transistor 674 is coupled to thesource of transistor 676. Likewise, the drain of transistor 676 iscoupled to the source of transistor 672, the gate of transistor 676receives OUT8X from third stage 606, and the source of transistor 676 iscoupled to the source of transistor 674. The drain of transistor 678 iscoupled to the common source node of transistors 674/676, the gate oftransistor 678 receives the NCK signal, and the source of transistor 678is grounded.

Fourth stage 608 is suitably configured to generate DQ_OUT9 in responseto DQ_OUT8, DQ_OUT8X, and the NCK signal. The operation of fourth stage608 is analogous to that of third stage 606. Fourth stage 608 samplesits input on the rising edge of NCK, and it maintains its output onnodes 680/682 until the falling edge of NCK, when OUT9 and OUT9X getprecharged to VTT. Fourth stage 608 precharges nodes 680/682 (and, inturn, OUT9 and OUT9X) to VTT when NCK is low and CK is high. When NCKtransitions from low-to-high, fourth stage 608 begins to quicklyevaluate its inputs (OUT8 and OUT8X) and, in response, adjust itsoutputs (OUT9 and OUT9X). On the other hand, when NCK transitions fromhigh-to-low, OUT9 and OUT9X both transition to the relatively high VTTvoltage. For example, when NCK transitions from low-to-high, if OUT8 isgreater than OUT8X, then fourth stage 608 will cause the voltage at node682 (OUT9X) to be pulled down to ground, while the voltage at node 680(OUT9) will remain at VTT. The converse applies if OUT8X is greater thanOUT8 when NCK transitions from low-to-high. The exemplary OUT9, andOUT9X signals in FIG. 9, which share a common time axis with the upperportion 626 of FIG. 9, illustrate the operation of fourth stage 608.

Third stage 606, in conjunction with fourth stage 608, converts a verysmall swing voltage (represented by DQ_OUT5 and DQ_OUT5X) into an outputsignal (represented by OUT9 and OUT9X) having a larger voltage swingthat is within the voltage domain of the processor. In this example,third stage 606 and fourth stage 608 transform an input having a 0.4volt swing into an output that varies between 0.0 volt (corresponding tologic low) and 1.0 volt (corresponding to logic high). OUT9 is fed tofifth stage 610, and OUT9X, which is unused in this embodiment, may befed to an output matching element (not shown) for purposes of balancingthe output of fourth stage 608.

Fifth stage 610 of DQ receiver 600 is suitably configured to function asa zero catcher and buffer that catches the falling edge of the OUT9signal, and quickly buffers and propagates it to the output to meettiming requirements. Fifth stage 610 is utilized in this embodimentbecause OUT9 and OUT9X have valid output values only when NCKtransitions from low-to-high. Otherwise, OUT9 and OUT9X are invalidbecause fourth stage 608 is precharging when NCK is low. Fifth stage 610maintains the valid output of OUT9 during the entire clock cycle, evenwhen NCK is high.

Referring to FIG. 8, this embodiment of fifth stage 610 includes,without limitation: two PMOS transistors 684/686; three NMOS transistors(reference numbers 688, 690, and 692); and two inverters 694/696. Thesource of transistor 684 is coupled to the VDDIO supply, the gate oftransistor 684 is coupled to the gate of transistor 688, and the drainof transistor 684 is coupled to a node 698. The gate of transistor 684is also coupled to fourth stage 608, to receive OUT9 as an input. Thesource of transistor 686 is also coupled to the VDDIO supply, the gateof transistor 686 is coupled to the gate of transistor 692, and thedrain of transistor 686 is coupled to node 698. The drain of transistor688 is coupled to node 698, the gate of transistor 688 is coupled to thegate of transistor 684, and the source of transistor 688 is coupled to anode 700.

The drain of transistor 690 is coupled to node 700, the gate oftransistor 690 receives the NCK signal, and the source of transistor 690is coupled to ground. The drain of transistor 692 is coupled to node700, the gate of transistor 692 is coupled to the gate of transistor686, and the source of transistor 692 is coupled to ground. Inverter 694has an input end coupled to node 698 and an output end coupled to thegate of transistor 686 and to the gate of transistor 692. Inverter 696has an input end coupled to node 698 and an output end from which theoverall output of DQ receiver (labeled OUT) is taken.

The operation of fifth stage 610 will be described with reference toFIG. 9 and, in particular, with reference to the OUT9 and OUT plots ofFIG. 9. Referring to the OUT9 plot, the zero bit periods are only validfor one half of the NCK cycle. When the NCK signal transitions fromlow-to-high, fifth stage 610 responds by passing the current OUT9 leveland maintaining it until the next low-to-high transition. For thisexample, the first zero bit in the OUT signal has been extendedthroughout one NCK clock cycle. At the second low-to-high transition ofNCK, OUT9 is at the relatively high 1.0 volt level. Accordingly, fifthstage 610 maintains the one bit in the OUT signal throughout that NCKclock cycle. At the third low-to-high transition of NCK, OUT9 is at therelatively low 0.0 volt level. Consequently, fifth stage 610 maintainsthe zero bit in the OUT signal throughout that NCK clock cycle,resulting in a zero, one, zero bit pattern.

The DQS receivers can also be implemented using transistor-basedelectronic circuits. In this regard, FIG. 10 is a circuit schematic ofthe first two stages, and FIG. 11 is a circuit schematic of the finalstage, of an embodiment of a DQS receiver 800. DQS receiver 800generally includes a first stage 802, a second stage 804, and a thirdstage 806. The general configuration and functionality of DQS receiver800 is consistent with that described above for DQS receiver 400 (FIG.5), and common features and aspects will not be redundantly described indetail here.

First stage 802 utilizes NMOS transistors configured as a common sourcecascode linear amplifier with resistor loading. In particular, firststage 802 includes, without limitation: four transistors (referencenumbers 808, 810, 812, and 814), a load resistor 816 coupled between thedrain of transistor 808 and VDDIO, and a load resistor 818 coupledbetween the drain of transistor 810 and VDDIO. Load resistors 816/818are selected according to the current generated by the transistors infirst stage 802, the value of VDDIO, and the desired voltage range ofDQS_OUT1 and DQS_OUT1X. As one non-limiting example, load resistors816/818 are each 1.6 kΩ in the illustrated embodiment. First stage 802also includes a tail current source 820, which may be realized as one ormore transistors.

The general topology and operation of first stage 802 is similar to thatdescribed above for first stage 602 of DQ receiver 600, and commonfeatures and aspects will not be redundantly described here in thecontext of DQS receiver 800. For first stage 802, the gate of transistor812 receives DQS_IN from the associated memory device, and the gate oftransistor 814 receives DQS_INX from the associated memory device. Morespecifically, the branch that includes transistor 810 generatesDQS_OUT1, and the branch that includes transistor 808 generatesDQS_OUT1X. Tail current source 820 is coupled between the common sourceof transistors 812/814 and ground. Notably, tail current source 820receives the same BIAS1 voltage that is used to bias the first stage ofall DQ receivers that are clocked with CK and NCK signals derived fromthe output of DQS receiver 800.

First stage 802 functions to transform the differential DQS input signalinto a differential output signal (DQS_OUT1 and DQS_OUT1X) having adesignated voltage swing from the positive supply voltage rail (VDDIO),regardless of the waveform characteristics (magnitude, rising/fallingedge rate, crossover voltage, etc.) of the DQS signal. For thisembodiment, the differential output signal swings between an uppersupply voltage (VDDIO) and a relatively low voltage that corresponds toVDDIO minus a swing voltage. The output voltage swing of first stage 802is controlled by the BIAS1 voltage via tail current source 820 and loadresistors 816/818. In operation, when DQS_IN is less than DQS_INX, thenDQS_OUT1X will be pulled to VDDIO and DQS_OUT1 will be VDDIO minus thedesired swing voltage (V_(SWING) _(—) _(DQS1)). Conversely, when DQS_INis greater than DQS_INX, then DQS_OUT1 will be pulled to VDDIO andDQS_OUT1X will be VDDIO minus V_(SWING) _(—) _(DQS1).

FIG. 12 is a diagram that depicts signals associated with the operationof DQS receiver 800. For this example, VDDIO equals 2.0 volts, VTTequals 1.0 volt, and VSS equals 0.0 volts. The point 822 represents thetime when DQS transitions, i.e., when DQS_IN crosses DQS_INX. Inresponse to this crossing, DQS_OUT1 and DQS_OUT1X transition after aslight delay caused by the practical operating characteristics of firststage 802. The point 824 represents the next transition of DQS. Inresponse to this subsequent crossing, DQS_OUT1 and DQS_OUT1X againtransition after a slight delay. FIG. 12 illustrates how V_(SWING) _(—)_(DQS1) represents a specified voltage drop from VDDIO.

The value of V_(SWING) _(—) _(DQS1) is programmed to be equal toV_(SWING) _(—) _(DQ1) multiplied by the ratio of the branch loadresistors in DQS receiver 800 and DQ receiver 600. The ratio is chosento properly bias DQS receiver 800 such that it propagates anon-distorted waveform to its third stage 806, while at the same timecausing DQ receiver 600 to experience a large enough voltage swing. Forthis particular example, the ratio is 1.6/2.4=0.667 and V_(SWING) _(—)_(DQ1) equals 0.4 volts. Thus, V_(SWING) _(—) _(DQS1) equals 0.267 voltsfor this example. Accordingly, the relatively high voltage level forDQS_OUT1 and DQS_OUT1X will be VDDIO (2.0 volts in this example), whilethe relatively low voltage level for DQS_OUT1 and DQS_OUT1X will be1.733 volts. As described in more detail below, the bias circuitry forfirst stage 802 is suitably configured to control the value of V_(SWING)_(—) _(DQS1), which represents the nominal expected voltage drop acrossload resistors 816/818 during operation. This scheme ensures thatV_(SWING) _(—) _(DQS1) is maintained regardless of the actual value ofVDDIO and regardless of variation between load resistors 816/818.

Second stage 804 of DQS receiver 800 is suitably configured to functionas a level shifter that downward shifts the dual-rail outputs of firststage 802 (DQS_OUT1 and DQS_OUT1X) for compatibility with third stage806. For this embodiment, second stage 804 is identical to second stage604 of DQ receiver 600, except for its input and output signals andassociated voltage levels. Moreover, second stage 804 is biased with thesame BIAS5 voltage that is used to bias the second stage of all DQreceivers that are clocked with CK and NCK signals derived from theoutput of DQS receiver 800. For the sake of brevity, and because theabove description of second stage 604 also generally applies here,second stage 804 will not be redundantly described in detail here.

Second stage 804 receives as inputs the DQS_OUT1 and DQS_OUT1X signalsfrom first stage 802. Second stage 804 performs level shifting on theseinput signals to generate output signals DQS_OUT5 and DQS_OUT5X.Referring again to FIG. 12, DQS_OUT5 equals DQS_OUT1 minus thedesignated shift voltage (V_(SHIFT) _(—) _(DQS2)), and DQS_OUT5X equalsDQS_OUT1X minus V_(SHIFT) _(—) _(DQS2). Although this example assumesthat V_(SHIFT) _(—) _(DQS2) equals 0.6 volts, which is the same asV_(SHIFT) _(—) _(DQ2), these two shift voltages need not be equal.Accordingly, the relatively high voltage level for DQS_OUT5 andDQS_OUT5X will be VDDIO minus V_(SHIFT) _(—) _(DQS2), or 1.4 volts forthis example. Similarly, the relatively low voltage level for DQS_OUT5and DQS_OUT5X will be VDDIO minus V_(SWING) _(—) _(DSQ1) minus V_(SHIFT)_(—) _(DQS2), or 1.133 volts for this example. Notably, second stage 804preserves V_(SWING) _(—) _(DSQ1) between DQS_OUT5 and DQS_OUT5X (0.267volts in this example).

Third stage 806 is suitably configured as a common source linearamplifier with resistor loading. This embodiment of third stage 806includes, without limitation: two PMOS transistors 826/828; a loadresistor 830 coupled between the drain of transistor 826 and ground; aload resistor 832 coupled between the drain of transistor 828 andground; and a tail current source 834, which may be realized as one ormore transistors. Here, tail current source 834 is a PMOS transistor.The source of transistor 826 is coupled to the source of transistor 828,and to tail current source 834. The gate of transistor 826 is coupled tosecond stage 804 of DQS receiver 800, and it receives the DQS_OUT5signal. The drain of transistor 826 is coupled to one end of loadresistor 830. Similarly, the source of transistor 828 is coupled to thesource of transistor 826, and to tail current source 834. The gate oftransistor 828 is coupled to second stage 804 of DQS receiver 800, andit receives the DQS_OUT5X signal. The drain of transistor 828 is coupledto one end of load resistor 832.

Tail current source 834 is coupled between the common source oftransistors 826/828 and VDDIO. Tail current source 834 receives theBIAS7 voltage (see FIG. 5 and FIG. 6) and responds thereto such thatthird stage 806 is biased so as to provide a particular voltage swing atits output. One output of third stage 806 (DQS_OUT7) is taken at thedrain of transistor 828, and the other output (DQS_OUT7X) is taken atthe drain of transistor 826. The output of third stage 806 is adifferential output that represents a downward shifted and slightlydelayed version of the differential input (DQS_OUT5 and DQS_OUT5X).

Third stage 806 operates in the following manner to transform itsdifferential input signal (DQS_OUT5 and DQS_OUT5X for this example) intoa differential output signal (DQS_OUT7 and DQS_OUT7X for this example)that swings between a lower supply voltage (e.g., VSS or ground) and arelatively high voltage that corresponds to VSS plus a swing voltage.When DQS_OUT5 is relatively low and DQS_OUT5X is relatively high, thenDQS_OUT7 will be pulled to ground (which corresponds to VSS or 0.0volts) and DQS-OUT7 will be at VSS plus a desired swing voltage(V_(SWING) _(—) _(DLL)). Conversely, when DQS_OUT5 is relatively highand DQS_OUT5X is relatively low, then DQS_OUT7X will be pulled to groundand DQS_OUT7 will be at VSS plus V_(SWING) _(—) _(DLL). The value ofV_(SWING) _(—) _(DLL) is chosen for compatibility with the DLL withwhich third stage 806 cooperates (see FIG. 2 and accompanyingdescription). For this embodiment, V_(SWING) _(—) _(DLL) equals 0.4volts. The bias circuitry for third stage 806 is suitably configured tocontrol the value of V_(SWING) _(—) _(DLL), which represents the nominalexpected voltage drop across resistors 830/832 during operation. Inaddition, using VSS as the low voltage reference may be a requirement ofthe DLL.

The DLL acts to provide a calibrated delay which will align the risingedges of the CK signal such that the rising edges occur in the middle ofthe eye defined by DQ_OUT5 and DQ_OUT5X. In FIG. 9, the point 702indicates this timing relationship of CK relative to DQ_OUT5 andDQ_OUT5X. This timing ensures that the sense amplifiers do not samplethe signals near a transition point. Accordingly, the DQS receiver andthe DLL can be cooperatively configured in this manner to generateCK/NCK with transition edge timing such that CK/NCK can be used tosample the output signals of multiple DQ receivers at a sampling timethat occurs when all of the output signals are well-settled. This timingcharacteristic ensures that the output signals are not sampled atpotentially invalid times when the output signals are transitioningbetween logic high and low states.

Referring again to FIG. 6, the DQ receivers and DQS receivers in acomputer system can cooperate with a programming architecture 500 thatgenerates the BIAS1, BIAS5, and BIAS7 voltages mentioned above.Furthermore, programming architecture 500 can be utilized to generatethe VCAS voltage mentioned above. Programming architecture 500 issuitably configured to automatically set the desired operatingconditions for any defined PVT corner. In particular, it generatesPVT-dependent bias voltages for the DQ receivers and DQS receiversdescribed above. In practice, the particular swing voltages, shiftvoltages, and common-mode output voltages (for both DDR2 and DDR3 modes)are optimized using simulation techniques prior to manufacturing of theprocessor device. Then, the optimized settings are stored as digitalBIOS-accessible registers with power-up and reset defaults converting toanalog bias voltages using digital-to-analog converters. These settingscan be reevaluated by characterization tests after manufacturing (e.g.,at each major production cycle). When the manufacturing process shiftsby some amount, the I/O receiver circuitry can remain optimized withoutany design changes by adjusting the register settings that influence theoperation of programming architecture 500. If the register settings havebeen shifted beyond the normal operating window, it is an indicationthat significant process changes have occurred. By detecting thedirection or trend of the register setting changes, one can determinethe portion of the receiver design that might need to be re-optimized orre-designed.

FIG. 13 is a circuit schematic of an embodiment of a first bias circuit900 suitable for use with a DQ/DQS receiver. Bias circuit 900 representsone suitable embodiment of a circuit that generates the BIAS1 voltagereferred to above. Bias circuit 900 is a replica circuit in that itcontains a replica of the circuit to which the BIAS1 voltage is applied.The replica circuit is placed in a feedback loop, and the output of thefeedback loop generates a replica bias voltage for the replica circuit.Ultimately, bias circuit 900 strives to duplicate this replica biasvoltage as the BIAS1 voltage.

Bias circuit 900 generally includes, without limitation: a referencevoltage generator 902; a receiver replica stage 904; a comparator 906; avariable current source 908 coupled to the output node of comparator906; and a current mirror 910 coupled to variable current source 908.Receiver replica stage 904 has an output node (node 912 in this example)coupled to the negative input node of comparator 906, and a bias inputnode coupled to a bias voltage node 915. In this embodiment, the biasinput node for receiver replica stage 904 corresponds to bias voltagenode 915. Receiver replica stage 904 is configured to mimic theoperation of first stage 602 of DQ receiver 600 (see FIG. 7) and/orfirst stage 802 of DQS receiver 800 (see FIG. 10). Unlike thecounterpart first stages 602/802, however, receiver replica stage 904uses fixed DC voltages at the gates of the two input transistors. Inparticular, VDDIO and VTT (which by definition is less than VDDIO) areused to force receiver replica stage 904 into a condition such that thereplica output generated at node 912 equals VDDIO minus the voltage dropacross a load resistance 914. Notably, receiver replica stage 904 isbiased with an internal bias voltage (labeled BIAS1_R) that is generatedby variable current source 908 at bias voltage node 915. In FIG. 13, thevoltage at node 912 is labeled OUT_REPLICA.

For this embodiment, about one-half of OUT_REPLICA is provided toanother bias circuit (to be described below). In FIG. 13, this halfvoltage is labeled V_(HALF). This half voltage may be obtained, forexample, by dividing load resistance 914 in half and accessing the nodebetween the two resistors. In this regard, load resistance 914 may berealized as a plurality of series resistors with various tap points ornodes therebetween. As depicted in FIG. 13, the voltage at one of thesetap nodes corresponds to V_(HALF). The exemplary embodiment describedhere includes a multiplexer, switch, or any suitable selection element919 that receives voltages corresponding to different tap nodes of loadresistance 914. Selection element 919 is programmed or otherwisecontrolled to select one of its input voltages for use as the VCASvoltage. The non-limiting example shown in FIG. 13 includes four inputvoltages, thus, selection element 919 can be digitally controlled usingtwo control bits. In practice, these control bits may be provided byprogram memory element 508 (FIG. 6).

In operation, bias circuit 900 strives to bias receiver replica stage904 with BIAS1_R such that the voltage of OUT_REPLICA is equal to theoutput generated by reference voltage generator 902. In FIG. 13, thisreference voltage is labeled VDDIO−V_(PROG1). In this embodiment,reference voltage generator 902 includes, without limitation: anadjustable current mirror 916 coupled between the positive input node ofcomparator 906 and a reference voltage (e.g., ground) node; avoltage-to-current element 918 coupled between VDDIO and adjustablecurrent mirror 916; and a load resistor 920 coupled between VDDIO andthe positive input node of comparator 906. The voltage-to-currentelement 918 provides a current proportional to an absolute voltage(provided by a bandgap reference circuit) divided by a resistorappropriately ratioed to load resistor 920. Reference voltage generator902 is suitably configured to set the reference voltage at node 922 to aparticular programmable amount, which can be varied to accommodatedifferent PVT conditions. In certain embodiments, the value of V_(PROG1)is dependent upon a BIOS-programmable digital code (e.g., a four-bitcode that provides sixteen different possible values) that enables thehost computer system to write to the registers of the I/O circuit toprogram the operation of bias circuit 900. Preferably, bias circuit 900employs a power-up default code value, which is determined bypre-silicon simulations, that results in the desired value ofVDDIO−V_(PROG1) Bias circuit 900 may use the programmable code value inan appropriate algorithm or formula to calculate the value of V_(PROG1).In one exemplary embodiment, the default code value results in areference voltage of VDDIO−0.560 volts, and that reference voltage isachieved by tuning adjustable current mirror 916 in an appropriatemanner. Thus, adjustable current mirror 916 is suitably controlled tocause a programmable voltage drop across load resistor 920, given thereference current supplied by the voltage-to-current element 918. Thereference voltage in one exemplary embodiment equals the bandgap voltagetimes the ratio of load resistor 920 to the resistor involtage-to-current element 918, times the current mirror ratio ofadjustable current mirror 916, which is programmed by the code mentionedpreviously.

As shown in FIG. 13, the reference voltage at node 922 (VDDIO−V_(PROG1))is applied to the positive input node of comparator 906, and the outputof replica circuit at node 912 (OUT_REPLICA) is applied to the negativeinput node of comparator 906. The feedback loop of bias circuit 900adjusts OUT_REPLICA until it equals VDDIO−V_(PROG1), within practicaltolerances. The comparator 906 generates a current control voltage atits output node, where the control voltage will vary depending upon acomparison of the reference voltage and OUT_REPLICA. In turn, thecurrent control voltage produced by comparator 906 will control theamount of current generated by variable current source 908. In thismanner, the current of variable current source 908 is dependent on thecurrent control voltage. The amount of current generated by variablecurrent source 908 influences the voltage of BIAS1_R, which is fed backas the bias voltage for receiver replica stage 904. Ultimately, when thefeedback loop reaches steady state, BIAS1_R will cause receiver replicastage 904 to be properly biased, resulting in the desired OUT_REPLICAvoltage.

Current mirror 910 is coupled to variable current source 908 such thatan output transistor element 924 generates the desired BIAS1 voltage(which mimics the internal BIAS1_R voltage). As mentioned above, theBIAS1 voltage is used to bias the counterpart receiver stage in thereceiver architecture. When implemented in a processor device, oneinstantiation of bias circuit 900 may support multiple DQ receivers andmultiple DQS receivers. In this regard, a single instantiation of biascircuit 900 may drive multiple instantiations of output transistorelement 924. Moreover, one instantiation of output transistor element924 may drive a plurality of DQ receivers and/or a plurality of DQSreceivers. Such a hierarchical bias circuit architecture may bedesirable to accommodate the practical needs, operating characteristics,and specifications of the processor device. In particular, it will saveDC current consumption over an embodiment that uses a replica biascircuit for each and every DQ and DQS receiver.

FIG. 14 is a circuit schematic of an embodiment of a second bias circuit926 suitable for use with a DQ/DQS receiver. Bias circuit 926 representsone suitable embodiment of a circuit that generates the BIAS5 voltagereferred to above. Bias circuit 926 is a replica circuit in that itcontains a replica of at least a portion of the circuit to which theBIAS5 voltage is applied. The replica circuit is placed in a feedbackloop, and the output of the feedback loop generates a replica biasvoltage for the replica circuit. Ultimately, bias circuit 926 strives toduplicate this replica bias voltage as the BIAS5 voltage.

Bias circuit 926 generally includes, without limitation: a referencevoltage generator 928; a replica circuit 930; a comparator 932; avariable current source 934; and a current mirror 936. Replica circuit930 is configured as a replica of one of the two branches of secondstage 604 of DQ receiver 600 (or, equivalently, second stage 804 of DQSreceiver 800). Replica circuit 930 need not replicate the entire secondstage of the DQ/DQS receiver because the second stage is not adifferential amplifier stage and, therefore, determining the biasvoltage for either identical half of the second stage will besufficient. Notably, replica circuit 930 need not employ a DDR2/DDR3mode switching transistor, as used by the second stage of the DQ/DQSreceiver (see FIG. 7 and accompanying description). Rather, bias circuit926 can be suitably controlled and programmed as needed to supporteither mode. Such control and programming is described in more detailbelow.

In contrast to second stages 604/804, replica circuit 930 uses a fixedDC voltage at the gate of an input transistor 938. As mentionedpreviously, this input voltage (labeled V_(HALF)) is obtained from biascircuit 900. Accordingly, bias circuit 926 is coupled to bias circuit900 in this embodiment. For the example described above, where the fullswing voltage is VDDIO−0.560 volts, V_(HALF) is equal to VDDIO−0.280volts. V_(HALF) is used here because the goal is to bias the secondstages of the DQ/DQS receivers such that the second stages produce adesired drop of the common mode voltage from input to output. This levelshifting characteristic will be influenced by the programmable referencevoltage (labeled V_(PROG5) in FIG. 14). In practice, V_(PROG5)represents the desired output level of a second stage when the signalpassing through the stage is at its crossing point, i.e., itshalf-voltage level. This is the reason why V_(HALF) is used in replicacircuit 930.

In operation, replica circuit 930 is biased with a voltage (labeledBIAS5_R) that is generated by variable current source 934. In FIG. 14,the voltage at the output node 940 of replica circuit 930 is labeledOUT_REPLICA. In operation, bias circuit 926 strives to bias replicacircuit 930 with BIAS5_R such that the voltage of OUT_REPLICA is equalto the output generated by reference voltage generator 928. In FIG. 14,this reference voltage is labeled VDDIO−V_(PROG5). The generalconfiguration, form, and function of reference voltage generator 928 issimilar to that of reference voltage generator 902. Accordingly,reference voltage generator 928 will not be redundantly described indetail here. Reference voltage generator 928 is suitably configured toset the reference voltage at node 942 to a particular programmableamount, which can be varied using a BIOS-programmable code toaccommodate different PVT conditions. In one exemplary embodiment, adefault code value results in a reference voltage of VDDIO−0.850 volts,and that reference voltage is achieved by tuning the adjustable currentmirror of reference voltage generator 928 in an appropriate manner. Thespecific programmability in this embodiment is similar to that of biascircuit 900, wherein the programmable code acts to control the currentmirror ratio that determines the reference voltage, given a referencecurrent input from a voltage-to-current element as described for biascircuit 900.

The remainder of bias circuit 926 is similar in configuration, form, andfunction to that described above for bias circuit 900. Accordingly, theremainder of bias circuit 926 will not be redundantly described indetail here. The feedback loop of bias circuit 926 adjusts OUT_REPLICAuntil it equals VDDIO−V_(PROG5), within practical tolerances. When thefeedback loop reaches steady state, BIAS5_R will cause replica circuit930 to generate the desired OUT_REPLICA voltage. In turn, an outputtransistor element 944 generates the desired BIAS5 voltage (which tracksthe internal BIAS5_R voltage). As mentioned above in the context of biascircuit 900, a single instantiation of bias circuit 926 may drivemultiple instantiations of output transistor element 944, and oneinstantiation of output transistor element 944 may drive a plurality ofDQ receivers and/or a plurality of DQS receivers.

FIG. 15 is a circuit schematic of an embodiment of a third bias circuit950 suitable for use with a DQS receiver. Bias circuit 950 representsone suitable embodiment of a circuit that generates the BIAS7 voltagereferred to above. Bias circuit 950 is a replica circuit in that itcontains a replica of at least a portion of the circuit to which theBIAS7 voltage is applied. The replica circuit is placed in a feedbackloop, and the output of the feedback loop generates a replica biasvoltage for the replica circuit. Ultimately, bias circuit 950 strives toduplicate this replica bias voltage as the BIAS7 voltage.

Bias circuit 950 generally includes, without limitation: a replicacircuit 952; a comparator 954; and two unity gain buffer amplifiers956/958. Replica circuit 952 is configured as a replica of third stage806 of DQS receiver 800. Unlike third stage 806, however, replicacircuit 952 uses fixed DC voltages at the gates of its two inputtransistors. In particular, VDDIO/2 and 0.0 volts (i.e., ground) areused to force replica circuit 952 into a condition such that the voltageat a node 960 can be monitored and compared by comparator 954. Inoperation, replica circuit 952 is biased with a voltage (labeledBIAS7_R) that is generated by comparator 954 and unity gain bufferamplifier 956. In FIG. 15, the voltage at the output node 960 of replicacircuit 952 is labeled OUT_REPLICA. Buffer amplifiers 956/958 areemployed to isolate the drive and stability requirements of comparator954 from the loads presented by the multiple circuits being driven byBIAS7. In operation, bias circuit 950 strives to bias replica circuit952 with BIAS7_R such that the voltage of OUT_REPLICA is equal to adesired voltage swing that is dictated by the operating characteristicsand preferences of the DLL being driven by to the third stage of the DQSreceiver. This desired voltage swing is labeled VSWING_(DLL) in FIG. 15.Thus, bias circuit 950 is suitably configured to generate BIAS7_R inresponse to the particular operating requirements of the associated DLL.For the embodiment described here, VSWING_(DLL) is about 0.4 volts aboveVSS.

The feedback loop of bias circuit 950 adjusts OUT_REPLICA until itequals VSWING_(DLL), within practical tolerances. When the feedback loopreaches steady state, BIAS7_R will cause replica circuit 952 to generatethe desired OUT_REPLICA voltage. In turn, an output transistor elementgenerates the desired BIAS7 voltage (which tracks the internal BIAS7_Rvoltage). As mentioned above in the context of bias circuit 900, asingle instantiation of bias circuit 950 may drive multipleinstantiations of the output transistor element, and one instantiationof the output transistor element may drive a plurality of DQS receivers.

While at least one exemplary embodiment has been presented in theforegoing detailed description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or embodiments described herein are not intended tolimit the scope, applicability, or configuration of the claimed subjectmatter in any way. Rather, the foregoing detailed description willprovide those skilled in the art with a convenient road map forimplementing the described embodiment or embodiments. It should beunderstood that various changes can be made in the function andarrangement of elements without departing from the scope defined by theclaims, which includes known equivalents and foreseeable equivalents atthe time of filing this patent application.

1. A bias circuit for a receiver architecture of a computer processor,the bias circuit comprising: a comparator having a positive input node,a negative input node, and an output node for a current control voltage;a variable current source coupled to the output node of the comparator,the variable current source being configured to generate a current thatis dependent on the current control voltage, and to generate an internalbias voltage at a bias voltage node; a receiver replica stage having anoutput node coupled to the negative input node of the comparator andhaving a bias input node coupled to the bias voltage node of thevariable current source, the receiver replica stage being configured tomimic operation of a counterpart stage in the receiver architecture bygenerating a replica output at its output node; and a programmablereference voltage generator coupled to the positive input node of thecomparator, the programmable reference voltage generator beingconfigured to generate a reference voltage that causes the receiverreplica stage to be properly biased, wherein the comparator receives thereference voltage and the replica output and generates the currentcontrol voltage based on a comparison of the reference voltage to thereplica output.
 2. The bias circuit of claim 1, further comprising acurrent mirror coupled to the variable current source, the currentmirror being configured to generate a bias voltage that mimics theinternal bias voltage, the bias voltage being used for the counterpartstage in the receiver architecture.
 3. The bias circuit of claim 1, theprogrammable reference voltage generator comprising: an adjustablecurrent mirror coupled between the positive input node of the comparatorand a reference voltage node; and a load resistor coupled between thepositive input node of the comparator and an upper supply voltage node,wherein the adjustable current mirror is controlled to cause a specifiedvoltage drop across the load resistor.
 4. The bias circuit of claim 3,wherein the specified voltage drop is programmable.
 5. The bias circuitof claim 4, wherein the specified voltage drop is programmable by abasic input/output system (BIOS) associated with the computer processor.6. The bias circuit of claim 1, wherein the reference voltage isprogrammable to accommodate different PVT (Process, Voltage,Temperature) conditions.
 7. The bias circuit of claim 1, wherein: thereceiver replica stage is a replica of a first linear receiver stage ofa data (DQ) receiver, the first linear receiver stage being configuredto receive a DQ signal and a reference voltage, and to generate a firstdifferential output signal in response to a comparison between the DQsignal and the reference voltage; and the reference voltage generated bythe programmable reference voltage generator causes the firstdifferential output signal to swing between an upper supply voltage anda voltage that corresponds to the upper supply voltage minus aprogrammable swing voltage.
 8. The bias circuit of claim 1, wherein: thereceiver replica stage is a replica of a second linear receiver stage ofa data (DQ) receiver, the second linear receiver stage being configuredto receive a differential input signal, and to generate a differentialoutput signal by shifting the differential input signal by aprogrammable shift voltage; and the reference voltage generated by theprogrammable reference voltage generator influences the programmableshift voltage.
 9. The bias circuit of claim 1, wherein: the receiverreplica stage is a replica of a first linear receiver stage of a datastrobe (DQS) receiver, the first linear receiver stage being configuredto receive a differential DQS signal associated with a plurality of data(DQ) signals, and to transform the differential DQS signal into adifferential output signal that swings between the upper supply voltageand a voltage corresponding to the upper supply voltage minus aprogrammable swing voltage; and the reference voltage generated by theprogrammable reference voltage generator influences the programmableswing voltage.
 10. The bias circuit of claim 1, wherein: the receiverreplica stage is a replica of a second linear receiver stage of a datastrobe (DQS) receiver, the second linear receiver stage being configuredto receive a differential input signal, and to generate a differentialoutput signal by shifting the differential input signal by aprogrammable shift voltage; and the reference voltage generated by theprogrammable reference voltage generator influences the programmableshift voltage.
 11. A receiver architecture for a computer processor, thereceiver architecture comprising: a first linear receiver stageconfigured to receive a first input, a second input, and a first biasvoltage, and configured to generate a first differential output signalin response to a comparison between the first input and the secondinput, the first differential output signal having a specified voltageswing that is influenced by the first bias voltage; and a firstprogrammable bias circuit coupled to the first linear receiver stage,the first programmable bias circuit being configured to generate thefirst bias voltage, wherein the first programmable bias circuit isprogrammable to provide overvoltage protection for transistors in thefirst linear receiver stage.
 12. The receiver architecture of claim 11,further comprising: a second linear receiver stage coupled to the firstlinear receiver stage, the second linear receiver stage being configuredto receive the first differential output signal and a second biasvoltage, and to generate a second differential output signal by shiftingthe first differential output signal by a shift voltage that isinfluenced by the second bias voltage; and a second programmable biascircuit coupled to the second linear receiver stage, the secondprogrammable bias circuit being configured to generate the second biasvoltage.
 13. The receiver architecture of claim 12, wherein the secondprogrammable bias circuit is programmable to provide overvoltageprotection for transistors in the second linear receiver stage.
 14. Thereceiver architecture of claim 12, further comprising: a third linearreceiver stage coupled to the second linear receiver stage, the thirdlinear receiver stage being configured to receive the seconddifferential output signal and a third bias voltage, and to transformthe second differential output signal into a third differential outputsignal having a swing voltage that is influenced by the third biasvoltage; and a third programmable bias circuit coupled to the thirdlinear receiver stage, the third programmable bias circuit beingconfigured to generate the third bias voltage.
 15. The receiverarchitecture of claim 14, wherein the second programmable bias circuitis programmable to provide overvoltage protection for transistors in thesecond linear receiver stage.
 16. A receiver architecture for a computerprocessor, the receiver architecture comprising: a linear receiver stageconfigured to receive a binary differential input signal and a biasvoltage, and configured to generate a binary differential output signalby shifting the binary differential input signal by a shift voltage thatis influenced by the bias voltage; and a programmable bias circuitcoupled to the linear receiver stage, the programmable bias circuitbeing configured to generate the bias voltage, wherein the programmablebias circuit is programmable to generate different bias voltages toaccommodate different PVT (Process, Voltage, Temperature) conditions.17. The receiver architecture of claim 16, wherein the programmable biascircuit is programmable to provide overvoltage protection fortransistors in the linear receiver stage.
 18. A receiver architecturefor a computer processor, the receiver architecture comprising: a linearreceiver stage configured to receive a binary differential input signaland a bias voltage, and configured to transform the binary differentialinput signal into a binary differential output signal that swingsbetween a lower supply voltage and a voltage corresponding to the lowersupply voltage plus a swing voltage that is influenced by the biasvoltage; and a programmable bias circuit coupled to the linear receiverstage, the programmable bias circuit being configured to generate thebias voltage, wherein the programmable bias circuit is programmable togenerate different bias voltages to accommodate different PVT (Process,Voltage, Temperature) conditions.
 19. A receiver architecture for acomputer processor, the receiver architecture comprising: a linearreceiver stage configured to receive a binary differential input signaland a bias voltage, and configured to transform the binary differentialinput signal into a binary differential output signal that swingsbetween a lower supply voltage and a voltage corresponding to the lowersupply voltage plus a swing voltage that is influenced by the biasvoltage; and a programmable bias circuit coupled to the linear receiverstage, the programmable bias circuit being configured to generate thebias voltage, wherein the programmable bias circuit is programmable toprovide overvoltage protection for transistors in the linear receiverstage.